Patent GB-1112589-A (GB)
Title: A wireless communication system utilizing a stationary satellite
IPC: H04Q11/04
Filed: 1966-02-21
Abstract: 1,112,589. Automatic exchange systems. NIPPON ELECTRIC CO. Ltd. 21 Feb., 1966 [27 Feb., 1965], No.7552/66. Heading H4K. An exchange system, in which inter-exchange connections are established by way of a geostationary satellite, provided with a frequencyshifting transponder and working in an assigned frequency band, has a data processing facility at a central control station at which a complete record is kept of the employment of all channels in the frequency band and of trunks at all exchanges communicating directly by way of the satellite so that allotment of channels and trunks is made at the central control station on application from an exchange setting up a connection. The central control station may also incorporate an exchange in the system and each of the exchanges communicating directly by way of the satellite may include an exchange working in its own local or national area. The satellite transponder receives signals in a frequency band F 2  and shifts them for transmission in a band F 1 . Each of the bands is divided into l frequency bands and one at least of these bands is time divided to provide n data channels. A request for the assignment of a channel is compiled at an originating exchange and is sent to the central control station by way of a permanently associated data channel. A store in the programme-controlled processor at the central control station contains a record of the state of all channels including the identity of the exchanges to which they may currently be allocated and also contains a record of the availability of all incoming and outgoing trunks of each exchange giving access to the satellite. In response to an assignment request the processor selects appropriate trunks and two idle channels for transmission in each direction. Messages are then compiled for transmission to the originating and terminating exchanges giving access to the satellite to enable controllers at these exchanges to set up the indicated trunks for reception and transmission on the chosen channels. The setting up and clearing down of speech circuits of this sort by way of the satellite is under the control of an operator at an international switchboard who can call in an operator at the distant international centre should this be necessary. No particular supervisory signalling system is preferred for the co-operation of the various registers involved in a connection but two conventional systems are discussed. Whenever a connection is cleared at an international centre a message must be compiled by its controller for transmission to the central control station so that the channel and trunk records can be brought up to date. If all channels are busy when a channel assignment request is received at the central control station, a message may be sent to the originating operator who may try the call at a later time; but it may be arranged that the processor at the central control station forms a queue of outstanding requests and sends to all exchanges an estimated waiting time which is updated at onesecond intervals. While the speech path may be established and be employed for supervision, and may need to be cleared if the called party is unobtainable, supervision may be conducted solely by way of data paths, the speech path being connected only when the called party replies or an operator responds to such reply. The time division multiplex made within a frequency-divided channel has a cycle time of 888 ms. with 74 time slots of 12 ms. each. Each time slot has a capacity of 72 bits. Each time division multiplex in the transmit frequency band has a corresponding multiplex in the receive frequency band. Because of the transmission times involved only the first half of each multiplex cycle is used to transmit and receive. The transmit time division multiplex of the central control station provides the fiduciary for the data channels in the system, the first time slot having twenty consecutive ones being transmitted as a synchronizing signal for the system. The relative phases of the send and receive time division multiplexes at the central control station, and at each exchange having direct access to the satellite, are such that corresponding time slots coincide at the satellite. To this end the synchronizing signal is received back from the satellite at the central control station and determines the phase of the receive multiplex, such determination automatically correcting for changes in height of the satellite. At each exchange data terminal reception of the synchronization time slot defines the receive time division multiplex phase. To fix the phase of the transmit time division multiplex and compensate for variations in height of the satellite it is necessary to monitor the transmission time from each exchange to the satellite and back and by a system of adders, comparators, and buffer stores to detect the differences between one cycle and the next to preserve a correct phase relative to the receive time division multiplex. Each start bit received is tested for persistence by counting 90 kc. pulses, the bit being treated as valid if the count reaches 8 which is about half the length of the bit. Codes are checkable and may be reverted for comparison with what has been sent before they are acted upon. The four-wire system employed for speech may be utilized for the purposes of a Telex system using time division multiplex data channels. Each exchange in this system requires to store the identity of two time slots for each connection and each time slot in the connection requires the service of a speed converter whereby two or three message codes are inserted in each time slot from a chain of buffer stores. Before transmission and after reception the messages are buffered on reperforators. A two-wire system may be employed for telegraph transmission, only one time slot and speed converter being needed for each connection. Allocation of time slots by the central control station can, in these instances, be made by way of the spare bits existing in the synchronizing time slot. Broadcast telegraphy is also said to be possible in the system.